[mlir] Add `const` qualifiers to `AffineMap` methods

The `AffineMap` class follows the same semantic as Type and Attribute.
It is immutable object, so it make sence to mark its methods as const.
Also part of its API is already marked as const, this change just make the API consistent.

Reviewed By: ftynse, bondhugula

Differential Revision: https://reviews.llvm.org/D96026